Is there a Scala pattern for collecting error messages from a sequence of independent computations?-CodePudding

Say I have a series of independent computations that need to be combined at the end:

def makeSauce():  Try[Sauce]  = Success(Sauce("marinara"))
def makeCrust():  Try[Crust]  = Failure(new RuntimeException("stale bread"))
def makeCheese(): Try[Cheese] = Failure(new IllegalArgumentException("moldy cheese"))

for {
  sauce  <- makeSauce()
  crust  <- makeCrust()
  cheese <- makeCheese()
} yield {
  Pizza(sauce, crust, cheese)
}

But if any of my computations fail, I want a comprehensive error message describing all of the failures, not just the first. The above code doesn't do what I want; it only shows me the first failure ("stale bread"). Instead, it should throw a super-error message combining both of the others, which could be a sequence of exception objects, sequence of string error messages, or even just one giant concatenated string.

Alternatively, instead of throwing the exception, it could be composed into a top-level Failure object:

(for {
  sauce  <- makeSauce()
  crust  <- makeCrust()
  cheese <- makeCheese()
} yield (sauce, crust, cheese)) match {
  case Success((sauce, crust, cheese)) => Pizza(sauce, crust, cheese)
  case Failure(e) => throw e
}

The for-comprehension and the Try here are just for illustration; a working solution doesn't have to use for or Try. However, obviously the above syntax is expressive and clean, and almost does what I want, so ideally the solution will stick to it as closely as possible.

How can I achieve something like this in Scala?

CodePudding user response：

scala.util.Try behaves in a fail-fast manner. In a for-comprehension context it executed sequentially and halts at the first failure.

What you need is a structure that aggregates all failures like cats.data.Validated or scalaz.Validation.

CodePudding user response：

Validated is what you are looking for. It is in Cats for example.

import cats.data.Validated
import cats.implicits.catsSyntaxTuple3Semigroupal

def valid(input: String) = if (input.toIntOption.isEmpty) Validated.Invalid(s"$input is not a number")
else Validated.Valid(input.toInt)

def sumThreeNumbers(one:String, two:String, three: String) = (valid(one), valid(two), valid(three)).mapN((oneInt, twoInt, threeInt)=> oneInt   twoInt   threeInt)

sumThreeNumbers("1","2","3")
sumThreeNumbers("1","two","xxx")
//results
val res0: cats.data.Validated[String,Int] = Valid(6)
val res1: cats.data.Validated[String,Int] = Invalid(two is not a number.xxx is not a number.)